I'm getting an error message saying "Port is unavailable"

This error message usually means there's another application taking over the port you are using to connect your printer. This application is likely to be Cura, Repetier Host, Markebot/Makerware, or any other app you use to control your printer.

Please close the application, make sure it is not running in the background, and your port should be released to be used by Authentise.